1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to a fixing device for thermally fixing a toner image on a recording medium.
2. Discussion of the Background
It is a common practice with an electrophotographic image forming apparatus to form an electrostatic latent image representative of a document image on the surface of a photoconductive element charged to a preselected polarity beforehand. The latent image is developed by a developing device located at a preselected position around the photoconductive element. Specifically, powdery charged toner is fed from the developing device to the phoconductive element in order to develop the latent image. The resulting toner image is brought to an image transfer station by the photoconductive element in rotation and transferred to a paper or similar recording medium also conveyed to the image transfer station. The toner image carried on the paper is fixed by a fixing device.
Generally, the fixing device is made up of a heat roller having a heat source therein and a press roller facing and pressed against the heat roller. The heat roller and press roller thermally fix the toner image on the paper being passed between the two rollers. Tie prerequisite with the heat roller is that it accurately contacts the image area of the paper and surely fixes the toner image on the paper. To meet this requisite, the heat roller is mounted on, e.g., the housing of the apparatus in such a manner as not to move in its axial direction or thrust direction. Means for restricting the movement of the heat roller in the thrust direction has been proposed in various forms in the past. Japanese Utility Model Laid-Open Publication No. 61-19265, for example, teaches a configuration in which a generally C-shaped stop ring in the form of a metal spring is resiliently fitted on the outer periphery of a heat roller. The stop ring abuts against a bearing and a support member supporting the heat roller so as to restrict the axial displacement of the roller.
Usually, the heat source disposed in the heat roller is implemented by a halogen lamp heater or similar heater capable of heating the roller by radiation. However, the problem with this kind of heat source is that a substantial interval is necessary between the time when the heater, i.e., the power switch of the apparatus, is turned on and the time when the surface of the heat roller reaches a preselected fixing temperature. That is, a long preheating time or warm-up time is necessary. In light of this, there has recently been proposed to form a heating layer heating in response to a current on the surface of the heat roller, as taught in, e.g., Japanese Patent Laid-Open Publication No. 9-127814. The heating layer directly heats the surface of the heat roller.
In a fixing device of the type using a heating layer as stated above, the surface temperature of the heat roller is usually controlled to a preselected value by the control of current supply to the heating layer. Specifically, the current supply to the heating layer 114 is controlled in accordance with the output of a thermistor or similar contact type temperature sensor contacting the surface of the heat roller. However, when the stop ring scheme is applied to the fixing device of the type accelerating the warm-up of the heat roller with the heating layer covering the surface of the roller, the stop ring and heating layer contact each other and bring about a leak. Particularly, when a leak occurs between the thermistor and the heating layer, current flows to the thermistor and prevents it from sensing temperature with accuracy.